Persistent infection has been observed for all ehrlichial species in a variety of animals and in humans. Understanding the mechanism of persistent ehrlichial infection will enable design of new therapeutic approaches to treat ehrlichial infections and vaccines to prevent the diseases. The following hypotheses are presented: 1) persistent infection is caused by antigenic variation and/or differential expression of the outer membrane protein (OMP) genes of Ehrlichia and 2) the OMPs confer immunity in the host against re-infection by the pathogen. The subject of this proposal is Ehrlichia chaffeensis which causes a newly emerging infectious disease, human monocytotropic ehrlichiosis. Persistent infection by E. chaffeensis has been documented in human and animals. A canine model will be employed to study the mechanism of this persistent infection. In specific aim 1, antigenic variation of E. chaffeensis gp120 will be analyzed in dogs infected by tick transmission. Canine blood will be obtained over a six month period, the gp120 gene will be amplified by PCR and sequenced and compared to the gene in the infecting isolate to determine the possible deletion or insertion of repeats in the gp120 gene. In addition, changes in the size and antigenicity of gp120 reisolated from infected canine blood will be examined. Specific aim 2 will entail analysis of the differential expression of the p28 multigene family. This will be accomplished by RT-PCR or epitope mapping. Aim 3 will involve testing of the protective immunity elicited in canines by immunization with recombinant OMPs, followed by challenge of immunized animals with E. chaffeensis. This will provide valuable information regarding the potential use of these antigens as subunit vaccines.